Kim Jong-nam, 45, was the oldest son of Kim Jong-ill, the current leader's father and predeccesor.
Initial reports by South Korean media say he was poisoned at Kuala Lumpar airport by two women said to be North Korean operatives.
Kim-Jong-nam had spent the best part of a decade in exile, having fallen out of favour with his late father.
The love child of an affair between the Surpeme Leader and an actress, he had been favourite to replace Kim Jong-il until he was arrested at a Tokyo airport in 2001 with a forged passport trying to get to Disneyland.
Kim Jong-nam has died in Malaysia
Kim Jong-nam enjoyed a reputation as a playboy
The incident caused great embarrassment to the leader, and Nam was exiled to China, where he lived in Macau on the south coast.
He had developed a reputation as something of a playboy, a fun-loving gambler with an appetite for alcohol and women.
He had previously spoken remarkably freely about his family's brutal regime, and in a 2012 interview with a Japanese journalist, called North Korea's dynastic succession "a joke to the rest of the world".
When his younger half brother Kim Jong-un – who he never met – was chosen to succeed his father, he predicted the regime would "not last long".
Malaysian police told Reuters an unidentified North Korean man died en route to hospital from a Kuala Lumpur airport.
A South Korean government source confirmed to the agency that it was Kim Jong-nam.
The police said the man's identity had not been verified.
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An employee in the emergency ward of Putrajaya hospital said a deceased Korean there was born in 1970 and surnamed Kim.
More to follow.